Modern digital systems which include computers, controllers, memories, and other digital modules will often make use of a variety of clock signals. Some of these clock signals are of the symmetrical type where the positive phase and the negative phase are in balance, and some are of the asymmetrical type clocks where there is a considerable difference between the positive phase and the negative phase in terms of time involved. Additionally, these modern digital systems which use these different types of clocks will also have variations in their frequency, their phase relationship, and the skew characteristic.
Generally, testing personnel are interested in the length (time period) of the pulse width of a signal transition such as when the signal changes from 0-1 into a positive phase, (this may be designated as t.sub.s) and when this particular pulse signal returns from 1-0, (this time period may be referred to as t.sub.e). These designations may be used to refer to the start point as t.sub.s and the end point as t.sub.e. Thus the pulse width would be defined as the period of time from the positive transition to the negative transition which would be the difference between the time moment t.sub.s and the time moment t.sub.e.
In the situation of "skew" measurement, here the situation may be defined as a condition where two signals are being compared as to the occurrence of a change, which may be called a "signal event". Then it is desired to see whether the signal event being compared between the two signals is exactly at the same time moment or there is a difference in time occurrence between the two compared signal events. If there is a difference between the time occurrence of the same type event in the two signals, then this difference in time period is known as "skew".
Thus if two signals, signal A and signal B, are being compared for a "0-1 transition event", and the first signal A has this particular event occur at a time t.sub.0 and the second signal B has the same event occur at time t.sub.1, then the difference between the two times of t.sub.0 and t.sub.1 is the measurement of the "skew" as to these compared signal events.
Thus, when a complex computer system, or I/O controller, or interface unit, or other type of digital module is built, it is necessary to know the conditions of each of the various clocks and their relationships to one another in order that proper system operation may be effectuated. In order to do this, it is necessary to know the condition of each of the various pulse widths of the clocks in the system, and it is also necessary to know how each of these pulse widths relate to one another, which information can be garnered by measuring the "skew" in a comparison between the same type of signal events as between the two signals being compared.
Heretofore, the common procedure was conducted by manually verifying each parameter of each clock with an oscilloscope where it was necessary for an operator to locate and isolate the signal and connect it to an oscilloscope so it could be observed. Considerable amounts of time and work effort were required to do this. For example, in one type of Burroughs computer system designated as the "A3 Series", there were some 23 clocks involved in the system which it was required to check for proper conformance to specifications. In this situation, it was required that a technicican or operator verify each individual clock signal and this could consume inordinate periods of time, sometimes more than an hour or two.
Additionally, since the measurements were based on the individual's perception and interpretation of the oscilloscope display, the test procedure was subjected to an individual or personal error level if the technician misperceived or misread the oscilloscope signals.
Thus the provision of an automated circuit apparatus was provided in order to eliminate the long time delay testing factor and to eliminate any possibility of individual perception errors.